How can I add stars to a picture?

There are loads of ways that you can improve your skies, but perhaps the easiest way is simply to place in a sky that you like over the top.

The Screen blend mode works well with skies because it erases all the dark shades in your picture and leaves just the light colours and most importantly, the stars. Place a picture of a night sky over your existing sky, set to the Screen blending mode and just mask so that it doesn’t overlap the rest of your scenery. It’s hard to replicate a starry night with brushes, so this is a good alternative.